An elliptical clause is a clause in which some words have been left out. Because of the pattern or logic of the entire sentence, it is clear what the missing words are.
An elliptical clause may be either independent or subordinate.
Example: Jessica had five dollars; Monica, three.
(The verb had was dropped from the second clause, but the meaning is still clear.)
For more on using elliptical clauses see Pronouns with Than or As and Commas with Elliptical Clauses